Ok here's your info-
There are about 100 different spawn points, all over the ledge in the fortress and in the mine.
The eggs can be dropped from anything hostile... usually in the mine.
The spawn time for each egg is random> I collected one egg once and it spawned again 1 second later.
Here is a helpful Video on collecting the eggs>>> it helped me get my nether drake mount in Just 5 days from pouncing on Eggs
http://ca.youtube.com/watch?v=2rumkxSmhrQ
Do Enjoy
P.S. i Chose the onyx one cause it iz teh pwn
>Edit<: Update. With more characters at 80 I've decided to farm on all of them and have found a couple more things that I forgot to mention previously. Eggs can be collected from nethercite deposits and netherdust bushes and the droprate off creatures has increased with patch 3.0.3 to about 3%. Again the way I got it on my alts was mainly the route shown in the video I posted above. Good Luck to all of you.
>Edit 2<: SPAWN INFO: Blizzard made a system where there is always a certain number of eggs in the area, meaning if there are 15 eggs always spawned, and people collect 2, 2 more will spawn elsewhere, or in the situation I had a year ago, I collected one and someone most likely collected one right after causing another to spawn right in front of me. They don't appear to be time based spawns, rather "come and find me and I'll show up somewhere else" spawns.